1. Field of the Invention
This invention provides a frequency discrimination capable of providing a substantially flat dispersion in a fiber optic system.
2. General Background
A frequency modulated laser source is modulated with digital data and the resulting output is passed through an optical frequency discriminator. The input to the discriminator is arranged to have a small amplitude modulation and sufficiently large frequency modulation. The frequency discriminator (FD) increases the extinction ratio of the output. In a fiber optic transmission system, which is limited by fiber dispersion, a frequency discriminated directly modulated laser transmitter (FDDML) provides a low chirp output in a compact package.
A frequency discriminator may be chosen to partially compensate for the dispersion in the transmission fiber and convert frequency modulated (FM) signal from a laser source to substantially amplitude modulated (AM) signal. The dispersion compensation aspect may be particularly applicable for high bit rate applications such as 10 Gb/s. To achieve a high frequency discriminator slope for high bit rates, a coupled multicavity (CMC) bandpass filter may be used on the edge of its transmission. A CMC may be made from depositing thin layers of alternating materials having high (H) and low (L) refractive indices. As illustrated in FIG. 1(a), a single cavity may be formed from an integer number of layers having a thickness of λ/2, sandwiched between a stack of λ/4 thick alternating high and low index layers, where λ is the design wavelength of light. FIG. 1(b) illustrates a CMC filter formed from a number of such cavities capable of producing a pass band and sharp slope transmission edges.
FIGS. 2(a) and 2(b) illustrate the filter transmission of a flat-top three-cavity CMC as well as its dispersion in units of ps/nm. In this FDDML, the modulated signal is spectrally aligned to be on either the positive slope 201 or the negative slope 202 of the discriminator. The discriminator may partially compensate for the dispersion of the fiber if the output signal 208 is spectrally aligned with the portion of the frequency discriminator spectrum 204, 205, 206, or 207 having dispersion opposite to that of the fiber. However, the dispersion changes sign near the transmission edges 201 and 202 so that care needs to be taken to ensure the spectral alignment. Besides the alignment, the three-cavity design may have rapid variation of dispersion with optical frequency that causes distortions of the optical signal due to third order dispersion. Third order dispersion is the derivative of the group velocity dispersion. Accordingly, there is a need to minimize the possibility of a misalignment and the distortion in the optical signal.
The flat-top filter also tends to cause the output intensity pattern to be distorted by overshooting or undershooting the 1 bits depending on the relative spectral alignment of the laser output to the filter transmission. A non-return-to-zero digital data stream is often shown on a sampling oscilloscope in the form of an “eye diagram,” as illustrated in FIG. 3, that is generated by superimposing the pulse train repeatedly on itself, each time shifting it by one bit period. FIG. 3 illustrates the eye diagram at the output of a flat top filter 300 for two different filter positions (b) and (c), showing the distortion.
FIG. 4 illustrates the output power of a directly FM modulated laser. The laser may be biased high above threshold and its bias current modulated to produce a digital signal having a 1-7 dB extinction ratio. As illustrated in FIG. 4b, due to line width enhancement, the optical frequency of the laser may undergo frequency excursion on the order of 2-15 GHz as the laser intensity is increased and decreased representing the digital bits. The frequency modulated laser signal may pass through a frequency discriminator, producing a substantially amplitude modulated signal with an extinction ratio greater than 10 dB. FIG. 4c illustrates the output of a positive slope discriminator, and FIG. 4d illustrates the output of a negative slope discriminator.
FIG. 4b illustrates that most lasers have the sign of frequency excursion as a blue shift for higher output intensities. The output of the positive slope discriminator has a higher power compared to the negative slope discriminator. However, the positive slope discriminator has a positive dispersion for the typical filter illustrated in FIG. 3 where it can only compensate for negative dispersion fiber. In order to compensate for the dispersion of standard fiber, which has positive dispersion (in units of ps/nm), the negative slope may be used near the passband, producing a reduced output power. As such, another objective of this invention is to design a CMC optical discriminator having negative dispersion on the positive slope side. A FDFD may be used for FDDML applications based on a distributed feed-back (DFB) laser.
Additionally the bandwidth of the filters is another parameter that has to be considered. A typical signal contains frequency components over a range corresponding to the data rate. For example a 10 Gb signal will contain frequency components in a 10 GHz bandwidth around the carrier frequency. As a result if the bandwidth of filters used is too narrow, this will affect the quality of the transmitted signal and in particular it will increase the rise and fall time which shows up in the eyes. FIG. 5 shows the impact on eyes when a step function is passed through two filters with a bandwidth of 12 GHz and 18 GHz, respectively. The higher bandwidth filter has a shorter rise time from the 20% to the 80% level (23 ps compared to 33 ps). To pass the SONET mask test, as required by telecom standards, a rise time that is smaller than 35 ps is required. After the signal is passed through the electrical 4th order Bessel-Thomson filter, the rise time increases further and to pass the mask test, the rise time before the filter should be below about 35 ps. This provides a limitation on the minimum bandwidth that a filter can have in this application. Typically, the bandwidth has to be at least as large as the data rate for good quality eyes to be generated. In addition to get high extinction at the output of the discriminator, the filter has to have a high slope of 1 to 2 dB/GHz.